It also has a well-documented origin, having been introduced by British/Indian sailors during the period of British-Japanese naval alliance. This also includes an explanation for why Japanese curry uses a roux base (the navy wanted to add vitamin B1-bearing wheat to the sailor's diet). What it doesn't include is an explanation for when, how, and why the apples became a key ingredient.
Per Chopstick Chronicles:
The ultimate Japanese curry rice secret ingredients “kakushi Aji”, which literally translates to “hidden taste”, are Apple and Honey. These are well-known Japanese curry rice ingredients among Japanese people so it’s not much of a secret anymore. Adding grated apple and honey gives the Japanese curry rice the signature sweeter flavour and is a staple for any Japanese mother’s home-cooked curry.
Apples are not used in Anglo-Indian curries that I've seen or been able to find online. So this leads to several related questions:
- Were apples added to standard Japanese curry when it was introduced to the 19th century navy?
- If so, were they added for nutritional reasons? Or were apples common in British curries of the time (the book "Curry" does not mention this, nor are apples mentioned in Mrs Beeton's)
- If the apples were added later, how did that come about? Was the Vermont Curry company responsible for it?
Thanks for any leads or ideas.